Utilization of balls for grinding is a known art. Among the known devices are ball mills, which comprise a container filled with grinding particles such as steel or ceramic balls and apparatus for stirring or agitating the balls. When particulate matter is introduced into the container, the agitation has the effect of reducing the size of the particles. The effectiveness of these devices are, in the main, limited to situations in which the particles to be ground are much smaller than the grinding particles and are suspended in a liquid.
A number of publications disclose the use in grinding of balls or other particles contained between two surfaces. In U.S. Pat. No. 276,418 there is disclosed a shoe and die for amalgamating pans in which "pulp and tailing from the battery . . . is ground together with quicksilver". In one of the embodiments described, grinding rollers are made to roll over a flat die.
U.S. Pat. No. 3,044,716 describes pulverizing mills in which balls are made to run in a circular raceway between two plates, the plates being entirely flat other than at the raceway. Material to be ground is fed into the center of a mill, passes through the raceway, and is ground before exiting on the periphery of the device.
U.S. Pat. Nos. 4,225,092 and 4,496,106 describe wet mills in which the grinding region is an annular volume defined between a stationary outer cylinder and a rotating inner cylinder. In both cases the annulus is filled with the material to be ground and the liquid carrier, to which are added generally spherical grinding elements having dimension much smaller than the thickness of the annulus.
U.S. Pat. Nos. 3,511,447 and 4,730,789 describe mills with attritive elements within variously configured grinding regions. The grinding regions are defined by moving and stationary surfaces and the attritive elements are much smaller than the smallest dimension of the grinding region. In both these patents there are intermediate (non-grinding) regions separating different grinding regions. During operation, the attritive elements are not present in the intermediate regions. In U.S. Pat. No. 3,511,447 the intermediate regions have a smallest dimension smaller than the smallest dimension of the elements; in U.S. Pat. No. 4,730,789 the regions have a smallest dimension slightly greater than the minimum dimension of the elements, with the elements being swept from these regions either by centrifugal or magnetic forces.
USSR Patent publication SU 445,466, discloses a mill for grinding ore, comprising an inner stationary truncated cone which is separated from an outer rotatable truncated cone by a plurality of crushing balls. Springs resiliently press the inner cone into engagement with the balls. Flexible material is provided on the surface of the inner cone facing the balls. In operation, material to be ground is fed through windows in a plate covering the mill and is thrown, due to centrifugal force, to the periphery of the outer cone, where it falls into a space between the two cones. The balls grind the material and the ground material then falls to the bottom and exits.
For dispersion of solids in liquids, where the mixture is highly viscous, as the solids and fluids used in the preparation of lithographic inks, three- or five-roll mills are used in which relatively large rollers, each rotating at a different speed, are almost in contact with one another, with a slight clearance between. Material fed into these mills is subjected to strong shear forces when passing between the rollers, the shear forces being operative both to reduce particle size and to improve dispersion of the solid material in the fluid.
These mills are relatively large and expensive and require considerable attention during operation. In particular, feeding, cleaning and material removal must be attended to. A further disadvantage of these mills is that the degree of dispersion achieved is often unsatisfactory, and consequently, the material must often be processed repeatedly until a desired fineness is achieved. Also, the mills are open to the atmosphere, resulting in substantial discharge of hydrocarbon vapors. Therefore, and in order to reduce health hazards and to avoid changes in ink characteristics, ink temperatures must be kept low.